How to decrease the polygon count on my mesh?
The simple answer is Mesh > Clean up > Decimate Geometry.

Then look in the bottom of the Tools panel to see the Decimate options and reduce the Ratio.



A better, non destructive option is to use the Decimate Modifier

You will find more options there. 



There seems to be an option which tries to keep UV maps intact which would be important if your mesh is using those.